Simply put, the “radio technology” is the wireless signal your phone uses. This also determines with what companies and where a phone will work. In the United States the big choice is between CDMA (Sprint and Verizon) and GSM (T-Mobile and AT&T).

This situation gets more complex than some people would like, but it’s helpful to understand. The biggest single thing to know is that you should go with the carrier that has the best service in your area–the rest is just details.

Your choice of a carrier determines the radio technology used to transfer voice and data across the cellular network. Unlike most carriers worldwide, and especially in Europe which settled around the common GSM (Global System for Mobile) radio technology, the U.S. carriers use either CDMA or GSM radio technologies. Verizon, Sprint, and a range of smaller carriers operate over the CDMA network, while AT&T and T-Mobile operate over the GSM network.

Because CDMA and GSM require different chipsets, you can’t use a Verizon phone on AT&T’s network and vice versa. For example, Palm’s CDMA Pre won’t work on AT&T’s network, and Apple’s GSM iPhone won’t run on Verizon’s network. There is also a third, far less widespread standard called iDEN (Integrated Digital Enhanced Network), developed by Motorola. iDEN network providers include SouthernLINC Wireless, numerous small companies covering specific markets and niche segments like airports, and Sprint’s Nextel division whose Nextel-branded phones run on iDEN as well.

3G: EV-DO versus HSDPA

CDMA (Code Division Multiple Access) is a hybrid 2.5G/3G technology that has a significant economic advantage over the rival GSM because it serves more phones using fewer cell sites. As a result, carriers operating over CDMA networks usually provide better coverage than their GSM counterparts, especially in rural areas where GSM carriers often can’t justify adding expensive GSM cell towers. On the other hand, GSM offers more phone choices and greater compatibility since most unlocked GSM phones work across the vast majority of GSM networks worldwide. In addition, GSM makes switching phones and carriers easy – you simply swap SIM cards when switching a carrier or put your existing SIM card into a new phone. Although there are two sub-types of CDMA in use today – CDMA2000 (2.5G) and EV-DO (3G) – CDMA2000 is considered abandoned as carriers like Sprint and Verizon focus solely on the speedier EV-DO. EV-DO Rev 0 and Rev A offer data speeds of up to 2.4Mbps and 3.1 Mbps, respectively. There’s also EV-DO Rev B that tops out at 4.9 Mbps and peaks at 14.7 Mbps, paving the way for bandwidth-hungry services like TV streaming. Both Verizon and Sprint have migrated their EV-DO Rev 0 networks to EV-DO Rev A.

Likewise, GSM also comes in 2.5G (EDGE) and 3G (UMTS/HSDPA) flavors. Operating on 850, 900, 1800, and 1900MHz bands, EDGE (Enhanced Data rates for GSM Evolution) provides four times the GPRS’ data throughput, but it typically tops at around 180Kbps. Then Cingular deployed EDGE in 2003 and both AT&T and T-Mobile have upgraded to a speedier 3G and its latest version HSDPA this year. Sometimes known as 3.5G, HSDPA (High-Speed Downlink Packet Access) operates on 850, 1900, and 2100MHz bands, offering downlink speeds of 1.8, 3.6, 7.2, and 14.0 Mbps. Its uplink counterpart dubbed HSUPA (High-Speed Uplink Packet Access) offers uplink speeds of up to 5.76 Mbps. Evolved HSPA, also known as HSP +, peaks at 42 Mbps and 22Mbps in downlink and uplink, respectively. Although both T-Mobile and AT&T announced plans to upgrade to HSPA+ by the end of 2010, 12 HSPA+ networks were already deployed across the world as of August 2009, with the fastest topping out at 28Mbps in Italy.

The U.S. carriers are falling behind their European counterparts in 3G deployment due to the size of the market and the country itself, which multiplies the high costs associated with obtaining new spectrum licenses and overlaying 3G at existing towers. Despite that fact that carriers and phone vendors cite maximum theoretical speeds in their marketing communication, note that these speeds are only attained in ideal laboratory conditions. Numerous real-life factors negatively impact theoretical maximums, like the size and the number of buildings and walls, terrain configuration, weather conditions, your distance from the cell tower, and network congestion. As a general rule of a thumb, count on achieving up to 50 percent of advertised speeds or less.

4G: Up to 1Gbps

The fourth generation of cellular wireless, commonly referred to as 4G, is set to succeed 2G and 3G networks beginning with next year. 4G radio technologies include the 802.16e mobile WiMax, HC-SDMA, and LTE (Long Term Evolution). 4G networks will provide data and streaming media at a much higher data rate, ranging from the Ethernet-grade 100Mbps between any two points in the world to 1Gbps if client and station are in relatively fixed position. This will enable bandwidth-consuming applications like HD broadcast, video calls, mobile TV, and more. 4G also enables more simultaneous users per call; meaning higher reliability and less dropped calls. Based on the ubiquitous IP protocol that powers the Internet, 4G networks will be interoperable with existing wireless standards and will offer global roaming across multiple networks. Most U.S. carriers are in early stages of upgrading their networks to 4G. Verizon is believed to be debuting the first commercial 4G network in the U.S. sometime in 2010, but most U.S. carriers will switch to 4G by the end of 2012.

The carrier choice determines the radio technology, the coverage, and handsets

Because each carrier opts for a specific radio technology, your pick of a carrier determines the coverage and the choice of handsets tailored for this network. Because of these dependencies, people in the U.S. are accustomed to picking their carrier first whereas users in other markets, especially European, normally choose their phone first and then their carrier, thanks to the fact that most overseas carriers operate over GSM. Most U.S. carriers sell the most popular phones on an exclusive basis, requiring you to pledge yourself to a multi-year service agreement before you can get the phone. The United States Federal Communications Commission (FCC) is expected to put an end soon to such so-called network exclusives.

What the future holds

Customers in the U.S. have long deemed carriers’ tactics that stifle competition unfair, arguing that carriers wield too much power. Consumer advocates say that carriers turned phone vendors into dumb manufacturers that are only asked to produce phones to carriers’ specs, namely the choice of the radio technology, phone features, and allowed network services. Application providers are hurt the most because carriers who run the network have the power to shut down phone apps and third-party network services that clash with their interests. For instance, most carriers block mobile Skype and other VoIP apps from using their networks to place cheap voice calls which replace carriers’ costly voice minutes. Because of this, application providers led by the most vocal of them – Google – have asked the FCC to intervene. The pressure from regulators and consumers combined with Obama’s proclaimed Net Neutrality has weakened carriers’ choke hold grip on the mobile industry, paving the way for innovative new services and applications that will run on mobile networks – regardless of carriers.